A disc drive is a data storage device used to store digital data. A typical disc drive includes a number of rotatable recording discs (optical or magnetic) which are axially aligned and mounted to a spindle motor for rotation at a high constant velocity. A corresponding array of read/write heads access tracks on the respective disc surfaces to write data to or read data from the discs.
A spindle motor is typically provided with a four-pole or six-pole motor configuration. Phase coils or windings are arranged about a stationary stator on a number of radially distributed stator poles. A rotatable spindle motor is provided with a number of circumferentially extending permanent magnets in close proximity to the stator poles. Application of current to the stator windings induces electromagnetic fields (called BEMF or back electromotive force) which interact with the magnetic fields of the magnets to apply torque to the spindle motor and induce rotation of the discs.
Controlling the spindle motor speed at which the spindle motor operates is critically important, as erroneous speed may result in an incorrect track being read or written. In existing disc drive systems, the BEMF generated in a single winding is typically measured and provided as a feedback signal to control the spindle motor speed. In response to the feedback signal, a spindle motor speed control system is used to modify the current flow in the stator windings to produce the necessary torque for rotating the spindle motor at a desired speed.